The invention relates to the field of intravascular catheters, and more particularly to a balloon catheter.
In percutaneous transluminal coronary angioplasty (PTCA) procedures a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient""s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter, having an inflatable balloon on the distal portion thereof, is advanced into the patient""s coronary anatomy over the previously introduced guidewire until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. See for example, U.S. Pat. No. 5,507,768 (Lau et al.) and U.S. Pat. No. 5,458,615 (Klemm et al.), which are incorporated herein by reference. Thus, stents are used to open a stenosed vessel, and strengthen the dilated area by remaining inside the vessel.
In conventional stent deploying balloon catheters, the balloon is made of essentially non-compliant material, such as nylon or polyethyleneterephthalate (PET). Such non-compliant material exhibits little expansion in response to increasing levels of inflation pressure. Because the non-compliant material has a limited ability to expand, the uninflated balloon must be made sufficiently large that, when inflated, the balloon has sufficient working diameter to compress the stenosis and open the patient""s passageway. However, a large profile non-compliant balloon can make the catheter difficult to advance through the patient""s narrow vasculature because, in a uninflated condition, such balloons form flat or pancake shape wings which extend radially outward. Consequently, the wings of an uninflated balloon are typically folded into a low profile configuration for introduction and advancement through the vessel. The wings are again produced upon deflation of the balloon following stent deployment within the patient. These wings on the deflated balloon are undesirable because they result in an increased balloon profile which can complicate withdrawing the catheter after stent deployment
Although stents have been used effectively for some time, the effectiveness of a stent can be diminished if it is not properly implanted within the vessel. For example, expansion of a balloon folded into a low profile configuration for introduction into the patient, can cause nonuniform expansion of a stent mounted on the balloon. The nonuniform expansion of conventional designs has resulted in the use of an elastic sleeve around the balloon and under the stent to distribute force from the expanding folded balloon to the stent uniformly, see for example U.S. Pat. No. 5,409,495 (Osborn), which is incorporated herein by reference. However, such sleeves may fail to completely prevent the nonuniform expansion of the stent, they increase the deflated profile upon insertion into the patient, and they complicate the assembly of the stent onto the balloon. Additionally, the final location of the implanted stent in the body lumen may be beyond the physician""s control where longitudinal growth of the stent deploying balloon causes the stent""s position on the balloon to shift during deployment. As the balloon""s axial length grows during inflation, the stent may shift position along the length of the balloon, and the stent may be implanted upstream or downstream of the desired location in the body lumen. Thus, balloons which have a large amount of longitudinal growth during inflation provide inadequate control over the location of the implanted stent.
Therefore, what has been needed is an improved catheter balloon. The present invention satisfies these and other needs.
One embodiment of the invention is directed to a stent delivery system with a stent deploying balloon formed of compliant material that uniformly expands the stent to properly implant the stent within the patient""s body lumen. Another embodiment is directed to a balloon catheter having balloon exhibiting semi-compliance or noncompliance, and a method of making the balloon.
The stent delivery system of the invention generally comprises a catheter having an elongated shaft with an inflatable balloon on a distal portion of the catheter and a stent disposed about the working length of the balloon. The balloon is formed of material compliant at least within a working range of the balloon, and which therefore provides for substantially uniform radial expansion within the working range. The compliant balloon material therefore expands substantially elastically when pressurized at least within the pressure range disclosed herein for use in inflating the stent deploying balloon of the invention. The compliant balloon material will generally be an highly elastic material. The term xe2x80x9ccompliantxe2x80x9d as used herein refers to thermosetting and thermoplastic polymers which exhibit substantial stretching upon the application of tensile force. Additionally, compliant balloons transmit a greater portion of applied pressure before rupturing than non-compliant balloons. Suitable compliant balloon materials include, but are not limited to, elastomeric materials, such as elastomeric varieties of latex, silicone, polyurethane, polyolefin elastomers, such as polyethylene, flexible polyvinyl chloride (PVC), ethylene vinyl acetate (EVA), ethylene methylacrylate (EMA), ethylene ethylacrylate (EEA), styrene butadiene styrene (SBS), and ethylene propylene diene rubber (EPDM). The presently preferred compliant material has an elongation at failure at room temperature of at least about 250% to at least about 500%, preferably about 300% to about 400%, and a Shore durometer of about 50A to about 75D, preferably about 60A to about 65D.
When the stent delivery balloon of the invention is pressurized, the balloon expands radially in a uniform manner to a working diameter. Because the balloon expands uniformly without unwrapping wings, it will uniformly expand a stent mounted on the balloon. The uninflated balloon does not require folding into a low profile configuration for insertion into the patient or the use of elastomeric sleeves used with conventional stent deploying balloons made from relatively non-compliant material. Similarly, the balloon of the invention should have a substantial elastic recoil so that it deflates into a smaller diameter with little or no wings. The undesirable flat or pancake shape wings which form when conventional stent deploying balloons are deflated are thus avoided. Additionally, minimal axial growth of the balloons during inflation provides improved control over the placement of the implanted stent in the body lumen. The compliant balloon results in improved abrasion and puncture resistance relative to the conventional non-compliant stent deploying balloons at least in part because there is little or no movement between the balloon and stent when the balloon expands radially. Moreover, due to the compliant nature of the balloon, there is a more highly efficient transfer of force to the stent than with the high pressure non-compliant conventional balloons which expend much expansive force to overcome rigidity (non-compliance) and to size the stent.
In another embodiment, the balloon catheter having a semi-compliant balloon generally comprises a catheter having an elongated shaft with an inflatable balloon on a distal portion of the shaft. The semi-compliant balloon is formed at least in part of a block copolymer, such as a polyurethane block copolymer. The term semi-compliant should be understood to mean a balloon with low compliance, which therefore exhibits moderate stretching upon the application of tensile force. The semi-compliant balloon has a compliance of less than about 0.045 millimeters/atmospheres (mm/atm), to about rupture, in contrast to compliant balloons such as polyethylene balloons which typically have a compliance of greater than 0.045 mm/atm. The percent radial expansion of the balloon, i.e., the growth in the balloon outer diameter divided by the nominal balloon outer diameter, at an inflation pressure of about 150 psi (10.2 atm) is less than about 4%. Another embodiment of the invention comprises a noncompliant balloon, preferably formed at least in part of a polyurethane block copolymer, which has a compliance of not greater than about 0.025 mm/atm.
In a presently preferred embodiment, the semi-compliant balloon is formed of a polyurethane block copolymer. Suitable polyurethane block copolymers include polyester based polyurethanes such as PELLETHANE available from Dow Plastics and ESTANE available from BF Goodrich, polyether based aromatic polyurethanes such as TECOTHANE available from Thermedics, polyether based aliphatic polyurethanes such as TECOPHILIC available from Thermedics, polycarbonate based aliphatic polyurethanes such as CARBOTHANE available from Thermedics, polycarbonate based aromatic polyurethanes such as BIONATE available from PTG, solution grade polyurethane urea such as BIOSPAN available from PTG, and polycarbonate-silicone aromatic polyurethane such as CHRONOFLEX available from Cardiotech. Other suitable block copolymers may be used including TEXIN TPU available from Bayer, TECOPLAST available from Thermedics, and ISOPLAST available from Dow.
One aspect of the invention is directed to a catheter balloon which is axially noncompliant. The terminology xe2x80x9caxially noncompliantxe2x80x9d should be understood to mean a balloon having a length which exhibits little or no axial growth during inflation of the balloon. The axially noncompliant balloon has an axial compliance of less than about 0.25 mm/atm, to about rupture. The length of the balloon increases by less than about 2.5% to about 20% over an inflation pressure range of about 60 psi (4 atm) to about 315 psi (21 atm), and by less than about 5% to about 15% within an inflation pressure range of about 90 psi (6 atm) to about 205 psi (14 atm). The balloon therefore avoids the trauma to the vessel wall caused when ends of an axially elongated balloon expand against a portion of the vessel wall.
The invention also includes a method of making a semi-compliant balloon. The method generally comprises extruding a tubular product formed at least in part of a block copolymer, such as a polyurethane block copolymer. The extruded tubular product is heated to a first elevated temperature and the outer diameter of the tubular product is expanded to a second outer diameter. While still under pressure, the expanded tubular product is heated at a second elevated temperature. The second elevated temperature is equal to or greater than the first elevated temperature. The expanded, heat-treated tubular product is then cooled to form the semi-compliant balloon. The tubular product is preferably heated to the first and second elevated temperatures by locally heating the tubular member with a heating member displaced along a length of the tubular product. The resulting balloons are semi-compliant, and axially noncompliant with low axial growth during inflation.
The semi-compliant block copolymer balloon of the invention provides improved performance due to the strength and softness of the balloon, with controlled expansion at relatively high pressures, and without the stiffness or poor refold characteristics of noncompliant balloons. Moreover, the low axial growth of the balloon during inflation provides improved control over the dilatation of a stenosis or implantation of a stent.
These and other advantages of the invention will become more apparent from the following detailed description of the invention and the accompanying exemplary drawings.